High Elastic Modulus Shafts and Method of Manufacture

ABSTRACT

High modulus turbine shafts and high modulus cylindrical articles are described as are the process parameters for producing these shafts and cylindrical articles. The shafts/articles have a high Young&#39;s modulus as a result of having high modulus &lt;111&gt; crystal texture along the longitudinal axis of the shaft/article. The shafts are produced from directionally solidified seeded &lt;111&gt; single crystal cylinders that are axisymmetrically hot worked before a limited recrystallization process is carried out at a temperature below the recrystallization temperature of the alloy. The disclosed process produces an intense singular &lt;111&gt; texture and results in shaft or cylindrical article with a Young&#39;s modulus that is at least 40% greater than that of conventional nickel or iron alloys or conventional steels.

TECHNICAL FIELD

This disclosure is directed to high elastic modulus shafts and othercylindrical articles. More specifically, methods of making high elasticmodulus shafts and other cylindrical articles are disclosed whichinclude seeding and casting single crystal investment castings in adesired crystallographic orientation and methods of hot working the samewhile preserving grain texture and limiting recrystallization. Theapproach in certain condition can also result with unusually hightensile and torsion strength materials.

BACKGROUND

Metallic materials generally have a crystalline form. Individual atomsof the material have a predictable relationship to their neighboringatoms which extend in a repetitive fashion throughout a particularcrystal or grain. The properties of such crystals vary significantlywith orientation.

Most metallic articles contain thousands of individual crystals orgrains. The properties of metallic articles in a particular directiondepend on an average orientation of the individual crystals which makeup the article. If the grains or crystals have a random orientation, thearticle properties will be isotropic, or equal in all directions. Thisis rarely the case since most casting, deformation, andrecrystallization processes produce at least some crystal orientation ortexture.

Crystals contain planes of atoms having particular spacings. Theseplanes are identified by Miller indices of the form (111), (110), (100)etc. X-ray measurements can be made and texture intensities can becharacterized as 1×, 5× random, etc., with 5× random indicating a moreintense texture than, for example, 2× random.

In rotating machines such as turbofans, auxiliary power generators, aswell as industrial gas turbines, the drive shafts are typically long andtransfer power generated by rotating turbine blades to the compressorblades as well as a large fan in the front of the engine to compress theair. In helicopter engines, the drive shaft drives the propeller.

In these and similar applications, the drive shafts are suspendedbetween bearings or extended from a single bearing so the shaft behaveslike a simple rotating beam, or a cantilever. The deflection of theshaft is inversely proportional to the axial stiffness or Young'smodulus of the material. The Young's modulus limits the naturalfrequency of the vibration and consequently the maximum rotation speedof the shaft. From this perspective, increasing the axial stiffness, andtherefore the Young's modulus, of the shaft is desirable as it enableshigher rotation speeds.

Stiff shafts are more forgiving to imbalanced rotating loads of bladesand disks in turbine engines. Stiff shafts also decrease the contact ofthe blade tips with the outer casing, thereby reducing leakage andimproving efficiency. Alternatively, an increase in stiffness allows fora longer shaft or increased spacing between the supporting bearings.Reducing the bearing assemblies not only can save weight, but may alsoprovide the design flexibility for accommodating a greater number ofturbine stages without interference of bearing assemblies. Piping fordelivering lubricant to the bearing assemblies can also be reduced.Other applications for stiffer shafts will be apparent to those skilledin the art.

Given that drive shaft failure is not acceptable, shafts are typicallyover-designed or based on a projected life. Further, drive shafts areunlikely to be made out of any other materials other than metals, withhigh ductility and toughness. Consequently, it is desirable to employ ashaft material having the highest possible Young's modulus to minimizedeflections. Among metals, except very high density tungsten andrhenium, Young's modulus of elasticity for most common polycrystallinesteel and nickel base alloys is approximately 30 Mpsi (207 GPa) at roomtemperature. Increasing the room temperature Young's modulus beyond 60Mpsi (414 GPa) may be possible with ceramic materials such as oxides andcarbides, but the brittle nature of these materials makes themunsuitable for shafts of rotating machines. Another approach is toproduce metal matrix composite with high strength fibers of for examplealumina or SiC. But this approach is also deemed risky due toinconsistent mechanical behavior associated with a coarse anduncontrolled fiber structure.

Therefore, there is a need for processes such as single crystalcastings, large single crystal castings, investment castings and hotworking of single crystal castings that produce shafts and otherarticles with a higher Young's modulus than currently available.

SUMMARY OF THE DISCLOSURE

This invention relates to a metallic component for a rotating machine,such as a drive shaft for a turbine or other cylindrical structure. Theshaft or cylinder are formed with a high modulus orientation <111> of aniron or nickel base alloy being aligned in the axial, i.e. primary,direction of the shaft or cylinder. It is understood that if the alloywere cobalt base with hexagonal crystal structure the high modulusdirection will be <0001> or the c-axis of the crystal, and if the alloywere molybdenum based the high modulus direction will be <100>.

In an embodiment, a method for producing a high elastic modulus shaftfrom an alloy is disclosed. The method includes providing a singlecrystal cylinder of the alloy. The single crystal cylinder has alongitudinal axis. The single crystal cylinder is also seeded so that ahigh modulus <111> direction is at least substantially parallel to thelongitudinal axis. The method also includes hot working the cylinder toachieve the desired size of the cylinder and heat treating the cylinderat a temperature below a recrystallization temperature of the alloy.

Another method of producing a high elastic modulus shaft from an alloyis disclosed. The method includes casting a single crystal cylinder fromthe alloy. The single crystal cylinder has a longitudinal axis. Thesingle crystal cylinder is seeded so that a high modulus <111> directionis at least substantially parallel to the longitudinal axis of thecylinder. The method also includes axisymmetrically hot working thecylinder to achieve a desired size of the cylinder at a temperaturebelow the recrystallization temperature of the alloy. Further, themethod includes heat treating the cylinder at a temperature below therecrystallization temperature of the alloy.

A high modulus shaft is also disclosed which includes a longitudinalaxis. The shaft includes a hot worked, limited recrystallized singlecrystal of an alloy having a high modulus <111> direction that is atleast substantially parallel to the longitudinal axis of the shaft.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a typical room temperature Young's modulus surfacefor nickel and iron base alloys in a single crystal with cubicstructure, which may be face centered, body centered, simple cubic orsome ordered variation such as B2.

FIG. 2 illustrates how a room temperature Young's modulus varies withcrystallographic direction in a typical complex nickel-base alloyrepresented with reference to a standard stereographic triangle.

FIG. 3 schematically illustrates a highly textured polycrystallinematerial with majority of the grains oriented along the high modulus<111> direction.

FIG. 4 schematically illustrates a prior art, random grain textured andprior art high modulus <111> grain textured shaft manufacturing process.

FIG. 5 schematically illustrates a disclosed method for producing a highmodulus <111> grain textured shaft, based on axisymmetric hot working ofa <111> oriented single crystal.

FIG. 6 illustrates a high modulus <111> oriented single crystalsuccessfully swaged with 61% reduction in diameter and confirmed toretain the high modulus <111>.

FIG. 7 illustrate a high modulus <111> oriented IN 718 single crystalingot successfully extruded with 50% reduction in diameter and confirmedto show 30% improvement in modulus over conventional polycrystallinematerial.

DETAILED DESCRIPTION

Investment casting is one of the oldest known metal-forming techniques.Beeswax was originally used to form investment castings. Today,high-technology waxes, refractory materials and special alloys are usedin investment castings, which provide accuracy, repeatability,versatility and integrity for a variety of metals and high-performancealloys.

The turbofan is a type of jet engine is widely used for aircraftpropulsion. The turbofan is basically the combination of two engines, aturbo portion which is a conventional gas turbine engine, and apropeller-like fan enclosed in a duct. The engine produces thrustthrough a combination of these two portions working in concert. The vastmajority of turbofans follow the same basic design with a large fan orcompressor at the front of the engine and a relatively small turbine orjet engine behind the large fan. There have been a number of variationson this theme, however, including dual compressors at the front of theengine and dual turbines at the rear of the engine. Other variationsinclude rear-mounted fans which can be easily added to an existing jetengine, or designs that combine a low-pressure turbine and a fan stagein a single rear-mounted unit.

Critical to the efficiency of turbofan engines is the maintenance ofminimum clearances between moving and stationary parts. The turbinedrive shaft is coupled to disks and blades for rotation and transmitspower from the turbine section to the compressor section of the engine.Efficient operation requires accurate location of the blades relative tothe casing. Therefore, it is of the utmost importance that the turbinedrive shaft be stiff and relatively free from deflection and vibration,although some vibration and deflection is unavoidable. The stresseswhich produce deflection and vibration of the drive shaft result fromthe engine operation and from externally applied loads resulting fromaircraft motion.

The Young's modulus can be selectively enhanced in one direction, byexploiting the high elastic anisotropy in nickel and iron base alloysingle crystals. As depicted by a schematic Young's modulus surface inFIG. 1, a room temperature elastic modulus approaching 44 Mpsi (304 GPa)can be achieved in a single crystal along the crystallographic direction<111>. Referring to FIG. 2, the variation of Young's modulus with allcrystallographic directions is described by a plot presented withreference to a stereographic projection triangle. From FIGS. 1 and 2, itis clear that Young's modulus varies by at least a factor of two, with alow value of about 18 Mpsi (124 GPa) in the crystallographic <100>direction to about 44 Mpsi (304 GPa) in the crystallographic <111>direction. This variation is exploited in components such as blades andvanes made out of cast single crystal where low elastic modulus isbeneficial for improving thermal mechanical fatigue and high elasticmodulus is beneficial in a vibratory environment where high cyclefatigue is a concern.

To enhance Young's modulus, it may be impractical to make the shaft outof cast single crystal with <111> direction parallel to the shaft axis.It is commonly assumed, based on experience, that isotropic finegrained, wrought materials are superior to cast materials from the standpoint of toughness and fatigue life. But this effect can also beexploited if a highly textured, polycrystalline, fine grained materialcan be produced, such that majority of the grains are oriented in <111>direction. Such material, depending on the degrees of the <111> graintexture in the axial direction of the shaft will on an average result inYoung's modulus between 35 to 40 Mpsi (241 to 276 GPa) at roomtemperature as set forth in U.S. Pat. No. 4,702,782. This concept isschematically depicted in FIG. 3.

Referring to FIG. 4, in order to achieve fine grained and wroughtstructure, conventional shafts 19, 20 are made by starting with acylindrical ingot 21, either cast or made by hot consolidation of powderof appropriate composition and having a large grain size 22. Such aningot 21 in solid form or as a hollow cylinder is hot worked at step 23using any of the axisymmetric methods such as swaging, extrusion, orrotary forge to achieve the final length and diameter and a somewhatsmaller grain size 24. The final shaft 20 is then appropriately heattreated at steps 25 or 26 to recrystallize fine grains and achievedesired microstructure 27.

The process of FIG. 4 typically yields a final product 19 or 20 withrandom grain texture 27 with an isotropic Young's modulus of about 30Mpsi (207 GPa). Some prior art processes are restricted to specificnickel alloy compositions, where the recrystallized grain texture is notrandom but preferentially forms the strong <111> texture. Such nickelalloy compositions typically have higher stacking fault energies. Thestacking fault energy is not an easily quantifiable parameter; the priorart process of FIG. 4 is restricted to alloys empirically observed toprovide a strong high modulus <111> texture such as high molybdenumcontaining nickel-base alloys or alloys based on precipitation of Ni₃Si.

The disclosed processes differ from the conventional processes of FIG. 4that provide a random grain texture shaft 19 or the shaft 20 with highmodulus <111> grain texture. As schematically shown in FIG. 5, thedisclosed process begins a single crystal cylinder 30 specifically castwith the axis 31 parallel to the high modulus <111> direction. This maybe achieved by directionally solidifying a nickel or iron base alloyingot 30 starting with a <111> oriented seed or any other single crystalseed 29 appropriately tilted to obtain <111> direction in the directionof solidification, which is the axis 31 of the cylinder 30. The castcylinder 30 is then appropriately heat treated at step 32 to soften thematerial and hot work the cylinder 30 to an appropriate reduction asshown by the intermediate product 33. The hot working step 32 ispreferably carried out below the temperature at which recrystallizationcan be triggered by heavy hot working.

As a result, a heavily worked cylinder 33 with a quasi-single crystal 34is obtained with a large amount of substructure and without destroyingthe maintenance of high modulus <111> orientation along the axis 31. Thecylinder 33 may not yield a recognizable X-ray or electron diffractionpattern reflecting the crystal symmetry, as it does for a cast singlecrystal. Nonetheless, the crystallographic nature of the cylinder 33 maybe confirmed by running X-ray diffraction intensity vs. Bragg angle scanand the elastic modus can be measured either by sound velocity ormechanical means.

The cylinder 33 is then subjected to a limited recrystallization processand direct age heat treatment at step 36 to produce a rod or cylinder 37with a Young's modulus in the <111> orientation ranging from about 37 toabout 40 Mpsi (˜255-˜276 GPa). Again, the process of step 36 isconducted at a temperature below the recrystallization temperature ofthe alloy.

The feasibility of method illustrated in FIG. 5 using axisymmetricswaging at a sub-scale level is shown in FIG. 6. Similarly thefeasibility of the process at a sub-scale production method such asextrusion is shown in FIG. 7. The process of FIG. 5 has beendemonstrated to work for several alloys, requiring adjustment of thepre-heat treatment to soften the material and the swaging temperature.Those well versed with nickel and iron base alloys can easily selectthese temperatures based on differential thermal analysis (DTA) ofliquidus, solidus and solvus temperatures for any given alloy. The hotworked high modulus single crystal then can be subsequently heat treatedin a controlled manner to minimize recrystallization to restore optimumbalance of engineering properties.

It is recognized that depending on the specific alloy composition, thepost-hot work heat treatment may be allowed to fully recrystallize thegrain structure if it happens to retain the <111> texture. However, thismethod can only be practiced for limited class of alloys. One disclosedmethod will be to limit the recrystallization so that high modulus <111>texture is largely retained as a quasi-single crystal with a largeamount of dislocation sub-structure and high dislocation density. Theheavily worked structure with a high dislocation density coupled withlow temperature precipitation in many nickel base superalloys as well asiron base alloys or steel will be sufficient to achieve a balance ofother engineering properties such as tensile strength, and fatigue life.Direct aging of IN718 is one example of such behavior, well known tothose well versed in the art of making structural components such asdiscs and shafts in jet engines.

Preliminary 1000° F. tensile test results of some swaged alloyspresented in the following table bear out the fact that not only such anhypothesis is correct but hot working single crystal holds even greaterpotential for improving tensile strength and consequently torsioncapability. Almost 30% increase in YS and UTS strength was achievedcompared to a typical wrought alloy. Traditionally it is assumed thatultimate torsion strength is 70% of UTS and on that basis, the resultsreflect the potential for improving torsion strength as well.

Comparison of 1000° F. Tensile Test Results. Temp (F.) Yield (ksi) UTS(ksi) % EL % RA Modulus (MSI) Hot Worked Waspaloy ® 1000 147.5 211.625.6 20.6 28 As swaged <111> PWA 1484 1000 222.5 256.9 6.5 6.5 40.9 1000178.7 236.8 8.7 11.8 41.6 Swaged + Aged <111> PWA 1484 1000 230.6 254.56.5 5.9 38.8 1000 223.9 226.3 3.3 4.9 42.8 Swaged + Aged <111> Udimet720 LI 1000 234.4 243.6 3.9 5 38.8 1000 226.6 249.5 8.2 6.3 37.8

Hot working of single crystal presents the added advantage ofdistributing the hot work homogeneously throughout the body of thematerial by eliminating the elasto-plastic incompatibility betweengrains, which is inevitable in polycrystalline material. There is plentyof experimental evidence to suggest that in polycrystalline materialsome grains owing to their unfavorable orientation and incompatibilitywith the neighboring grain do not respond to the hot work, therebybringing down the average enhancement in strength due to work hardening.Coarser the starting grain size, worst is this situation. In case ofsingle crystal this situation is eliminated provided uncontrolled shearalong limited slip system is not allowed by well selected hot workingstrategy.

Furthermore the fact that same level of strengthening was achieved in asingle crystal alloy PWA 1484 and a typical wrought alloy Udimet 720 LI,by hot working <111> oriented single crystal, clearly suggests that thestrength enhancement was purely due to work hardening or increase inuniform dislocation density.

Those well versed in the art will immediately recognize that theproposed approach could be used to further enhance the strength in solidsolution strengthened alloys containing no precipitate structure such asHastealoy-X, and such an approach may provide a better opportunity toenhance strength without loss of ductility.

Also, if retention of high modulus were not required, then in case ofprecipitation hardened alloys, additional strength enhancement may beachieved by allowing the material to recrystallize and addingprecipitation strengthening as well as fine grain strengthening.

Starting with a highly oriented high modulus (˜40-44 Mpsi; ˜276-304 GPa)direction with <111> oriented single crystal cylinder 30 leaves a lot oflatitude for retaining the high modulus for alloys including those thattend to form random grain texture. This is distinctly different fromprior art approaches where the starting cylinder 21 has intermediatelevel of elastic modulus (˜30 Mpsi; ˜207 GPa), followed byrecrystallization of the random texture (see steps 25, 26 of FIG. 4),which works only for certain alloy compositions.

It is realized that for some specific alloys, it may be moreadvantageous to start with single crystal in different orientation suchas <110>, <112>, <100> or <123>, or even columnar grain material, if therecrystallization texture ultimately leads to strong <111> texture. Thedisclosed process suggests a novel way of starting with directionallysolidified single crystal 30 or columnar grain material 30, in contrastto conventional methods for making high modulus shaft that start with acast or consolidated particle ingot 21. For most classes of nickel andiron base alloys, <111> oriented single crystal may be the practicalapproach.

In this context it is also recognized that some iron base alloys andsteels where strength could be derived from martensitic transformation,the end orientation or grain texture, may systematically shift from thestarting orientation owing to habit plane relationship between theparent phase and resulting martensitic phase. In such a situationdepending on the combination of strength and modulus desired, deviationfrom the <111> orientation may be desirable.

It is also recognized that unconventional hot working processes such astwist extrusion and equal channel angular extrusion also may be used toenhance strength, and may present added advantage in terms of selectionof starting single crystal orientation. For example a combination ofsuch technique may allow to convert a starting single crystal in <100>orientation to a high modulus <111> textured material, presenting amanufacturing advantage at the casting stage by eliminating seedrequirement, as <100> is a natural growth direction.

INDUSTRIAL APPLICABILITY

The disclosed process is a combination of process steps. First, a singlecrystal casting can be seeded to produce a desirable crystallographicorientation. Large single crystal castings, approaching one foot indiameter and several feet in height can be cast by conventionalinvestment casting or zone melting. Further, hollow single crystalcylinders can also be cast using ceramic or refractory metal cores.Single crystal casting of selected orientation can be axisymmetricallyhot worked followed by limited recrystallization or direct age treatmentusing various techniques to achieve desired grain texture. Optimumprocessing parameters in all steps may differ from alloy to alloy, butnone of the processes are fundamentally expected to be limited to aspecific composition of nickel or iron base alloys.

Nickel base and iron base (steel) shafts generally possess hightoughness and high ductility. Achieving high elastic modulus in theseclasses of materials makes their application as a high modulus shaftmaterial much less riskier than high modulus composite shafts. Changingthe grain texture has only a secondary influence on the plastic behaviorof the material.

What is claimed is:
 1. A method of producing a high elastic modulus shaft from an alloy, the method comprising: providing a single crystal cylinder of the alloy, the single crystal cylinder having a longitudinal axis, the single crystal cylinder being seeded so that a high modulus <111> direction is at least substantially parallel to the longitudinal axis; hot working the cylinder to achieve a desired size of the cylinder; heat treating the cylinder at a temperature below a recrystallization temperature of the alloy.
 2. The method of claim 1 wherein the alloy is a nickel based alloy.
 3. The method of claim 1 wherein the alloy is an iron based alloy.
 4. The method of claim 1 wherein the alloy is steel.
 5. The method of claim 1 wherein the hot working comprises axisymmetrically hot working the cylinder.
 6. The method of claim 1 wherein the heat treating is direct age heat treating.
 7. The method of claim 1 wherein a Young's modulus in the <111> direction is greater than about 37 Mpsi (255 GPa).
 8. The method of claim 1 wherein a Young's modulus in the <111> direction ranges from about 37 to about 45 Mpsi (from about 255 to about 310 GPa).
 9. The method of claim 1 wherein the cylinder is solid.
 10. The method of claim 1 wherein the cylinder is hollow.
 11. A method of producing a high elastic modulus shaft from an alloy, the method comprising: casting a single crystal cylinder from the alloy, the single crystal cylinder having a longitudinal axis, the single crystal cylinder being seeded so that a high modulus <111> direction is at least substantially parallel to the longitudinal axis of the cylinder; axisymmetrically hot working the cylinder to achieve a desired size of the cylinder at a temperature below a recrystallization temperature of the alloy; heat treating the cylinder at a temperature below the recrystallization temperature of the alloy.
 12. The method of claim 11 wherein the alloy is a nickel based alloy.
 13. The method of claim 11 wherein the alloy is an iron based alloy.
 14. The method of claim 11 wherein the alloy is steel.
 15. The method of claim 11 wherein the heat treating is direct age heat treating.
 16. The method of claim 11 wherein a Young's modulus in the <111> direction is greater than about 37 Mpsi (255 GPa).
 17. The method of claim 1 wherein a Young's modulus in the <111> direction ranges from about 37 to about 45 Mpsi (from about 255 to about 310 GPa).
 18. A high modulus shaft comprising: the shaft having a longitudinal axis; the shaft comprising a hot worked, limited recrystallized single crystal of an alloy having a high modulus <111> direction that is at least substantially parallel to the longitudinal axis of the shaft.
 19. The shaft of claim 18 wherein a Young's modulus of the shaft in the <111> direction is greater than about 37 Mpsi (255 GPa).
 20. The shaft of claim 18 wherein a Young's modulus of the shaft in the <111> direction ranges from about 37 to about 44 Mpsi (from about 255 to about 310 GPa).
 21. A method of producing a high strength and or high torque resistant shaft from an alloy, the method comprising: providing a single crystal cylinder of the alloy, the single crystal cylinder having a longitudinal axis, the single crystal cylinder being seeded so that a high modulus <111> direction is at least substantially parallel to the longitudinal axis; hot working the cylinder to achieve a desired size of the cylinder; heat treating the cylinder at a temperature below or above a recrystallization temperature of the alloy to maximize the strength.
 22. The method of claim 21 wherein the alloy is an iron based alloy.
 23. The method of claim 21 wherein the alloy is steel. Such steel may be ferritic, austenitic, martensitic or strengthened by precipitation and commonly used for all wrought and cast structural applications.
 24. The method of claim 21 wherein the hot working comprises axisymmetrically hot working the cylinder. 